It is known that amine oxides are useful materials, those that are most attractive commerically being the mixed amine oxides, i.e., amine oxides having at least one long-chain group and at least one short-chain group attached to the amino nitrogen. These oxides are used in many formulations in which their surface activity is an attribute, e.g., laundry detergents, rinses, and dryer sheets; shampoos and hair conditioners; soaps, and other personal hygiene products.
As taught in U.S. Pat. No. 4,748,275 (Smith et al.-I) and the references discussed therein, there are many known methods of preparing amine oxides by reacting tert-amines with aqueous hydrogen peroxide. The syntheses most commonly employed are the aqueous processes utilizing sufficient water to provide the products as aqueous solutions, e.g., the processes of U.S. Pat. No. 4,247,480 (Murata et al.) and European Patent Application 0307184 (Bauer et al.) in which carbon dioxide is used to promote the reaction. Less commonly, the amine oxides are prepared in organic solvents, as in U.S. Pat. No. 3,776,959 (Stalioraitis et al.).
The aforementioned solvent processes are quite satisfactory for the products which are to be used in applications in which their water or organic solvent content can be tolerated. However, the utilization of these processes necessitates the performance of after-treatments, such as spray-drying or evaporation, when the amine oxides are intended for use in applications, such as dry solid laundry detergent formulations, in which the presence of the solvent cannot be tolerated.
Smith et al.-I teach that the use of a temperature high enough to maintain the product in a molten state permits some amine oxides to be prepared in the solid form that makes them more desirable than the dissolved oxides for some purposes.
Copending application Ser. No. 07/591,426 (Smith et al.-II) discloses a process in which stirrability of the reaction mixture is maintained by conducting at least the latter part of the tert-amine/hydrogen peroxide reaction in an organic solvent in which the amine and amine oxide are soluble at the reaction temperatures but in which the amine oxide is insoluble at a lower temperature, thus permitting a relatively easy recovery of the amine oxide in solid form. In some respects this process is more attractive than the process of Smith et al.-I. However, when it is used to prepare a substantially pure amine oxide, it requires the use of centrifugation, crystallization, and drying steps which add to its cost.